To become a mediator, you must attend a mediation training course.
The following organisations offer CMC Registered training courses which can lead to CMC Registered status (see here [link] for all of the registration requirements).
Academy of Experts
Align Mediation (though not the distance learning course)
Globis Mediation Group
London School of Mediation
National Dispute Resolution
Phoenix Dispute Solutions
Regents University London
Royal Institute of Chartered Surveyors (RICS)
Society of Mediators
University of Central Lancashire